1. Field of the Invention
The present invention relates to a coating unit for coating a surface of a treatment body such as a semiconductor wafer with a coating solution such as a resist solution.
2. Description of the Related Art
In a photolithography process of semiconductor device fabrication processes, for example, first, a resist coating treatment is performed to form a resist film on a surface of a semiconductor wafer (referred to as "a wafer" hereinafter) Next, after being resist-coated, the wafer is exposed and then developed.
A coating unit for performing a resist coating treatment is now taken up. A conventional coating unit has a structure shown in FIG. 18. Specifically, a resist solution kept in a resist tank 201 is pressed out to a nozzle with a pump 202. Meanwhile, timing for discharging the resist solution from a nozzle 205 is controlled by opening and closing a flow path of the resist solution with a valve 204 disposed between the pump 202 and the nozzle 205.
FIG. 19 is a schematic sectional view of an air operation-type valve 204 used in the aforementioned coating solution supplying mechanism.
The valve 204 used here is an air operation-type valve which makes opening and closing operation through compressed air.
In the valve 204, a cylinder 212 is disposed between a suction side flow path 210 and a discharge side flow path 211. Within the cylinder 212, a rod 213 is held to be movable vertically in FIG. 19 through diaphragms 214 and 215. The rod 213 is biased downward in FIG. 19 by a spring 216. An air chamber 217 is disposed in an upper portion of the cylinder 212 and under the diaphragm 214. The air chamber 217 is communicated with the outside through an operation port 217a. An air compressor (not shown) is connected to the operation port 217a by way of a solenoide valve (not shown).
When the solenoide valve is closed and compressed air is not sent from the air compressor, space between a lower end portion of the rod 213 and a gate portion 218 is closed by pressing force of the spring 216.
In order to work the valve 204, the solenoide valve is opened and compressed air from the air compressor is sent to the air chamber 217, whereupon the diaphragm 214 is transformed by force of the compressed air and the rod 213 is pushed up. Between the lower end of the rod 213 and the gate portion 218, space is formed so that the suction side flow path 210 and the discharge side flow path 211 are communicated with each other.
Incidentally, a coating solution of low viscosity is advantageous, because production cost can be lowered, a treatment body with a large area can be coated therewith, and the like. Therefore, a low viscosity-type coating solution containing more solvent or surface active agent than conventional coating solutions is mainly being used.
A low viscosity-type coating solution has, however, lower surface tension as compared with conventional coating solutions of relatively high viscosity. Thus, the low viscosity-type coating solution is easy to drip down from a forward end of a nozzle, and bubbles easily arise in the coating solution by air which enters from the forward end of the nozzle. Due to the aforementioned dripping and bubbles, thickness of a coating film changes and the quality of a treatment body easily deteriorates.
To prevent the aforementioned disadvantages, it is necessary to work a pump and a valve with a good response, to accurately open and close the valve in timing that dripping of a resist solution or bubbles do not arise, and to slowly perform the operation when the valve is closed.
A conventional air operation-type valve, however, has a structure that a valve body is driven through compressed air. Accordingly, operation is accompanied by delay time so that it is difficult to accurately control the time for working an opening and closing valve. Moreover, a conventional solenoide valve has difficulty in controlling the working speed at the time of opening and closing it. As a result, there appears a disadvantage that the occurrence of dripping or bubbles can not be completely prevented when a low viscosity-type coating solution is used.